A floatable bottom-founded caisson is disclosed that is suitable for use in ice-covered waters. The annular caisson structure has a frusto-conical upwardly tapered inner wall which reduces skin friction between itself and the core fill material. By the application of heat and the use of insulation, freezing of the core fill material is prevented; this feature, together with the tapered wall, enables the caisson to be easily raised and moved to a new position. The outer, or perimetrical, wall is inclined to aid upward fracturing of the ice impinging on it. The caisson carries a deck structure for the support of offshore drilling or other equipment.